As a previous owner of the 18-270, it is not a bad lens for what it is, but of course you will get better quality if you get multiple lenses covering the same range. Over the 18-50mm range it is about as sharp as the others in the middle, although it does degrade more in the corners. Trading off that, of course you get a massive zoom range. Personally once you get to 200mm+ on that lens it does lose sharpness.

Tamron makes quite decent glass, and some quite knowledgeable people say their vibration control is as good or better than the big boys' versions. Lens creep can be a bother, if the camera is oriented vertically, but that is true of all superzoom SLR lenses, including Nikkor and Canon. My wife and I have personal experience with pre-VC Tamron 18-200mm lenses on Nikon and Canon cameras. She likes the superzoom concept; I do not. She accepts lens creep and sharpness compromises at each end of the focal range in exchange for not ever having to swap lenses in the field.

True, there is a latch to prevent creep, but it only latches at the 18mm setting.

If you have a Canon, be SURE to buy the right version! Di is for full-frame Canons, Di II is for cropped-frame Canons. Di will fit cropped-frame Canons, but the electronics will not "talk" to each other.

Superzooms are not without merit! My wife can get a shot at a distant bird or something, while I miss some opportunities due to having the wrong lens mounted. On the other hand, I am more likely to have my camera in hand, with a fixed-focal-length 50mm making the whole package handier.

Just to be clear, my wife is much more experienced and talented than I am.